This invention relates to a heterodyne stage for high-frequency receivers of a type including a mixer with an output thereof having a broadband terminating separating filter and an intermediate-frequency filtered.
All materials from the priority application, German Patent Application 199 21 438.7, filed Mar. 10, 1999, is incorporated by reference herein.
Broadband terminating separating filters are known as so called diplexers. A broadband termination of a mixer output, independent of an input resistance of an IF filter, in a transmission region as well as in a cutoff region, has been, until now, possible only with relatively high circuit complexity, using several quartz crystal filters and broadband phase rotation circuits (Meinke/Grundlach, Taschenbuch der Hochfrequenztechnik, 5edition, page Q22, FIG. 8).
An object of this invention is to provide a heterodyne stage in which, with minimal circuit complexity, a mixer is continuously terminated with a system impedance, and which has minimal transmission attenuation.
This object is achieved in a heterodyne stage of the type mentioned in the opening paragraph above by using a resistance having a value of a system impedance arranged between a separating filter and an intermediate-frequency filter, with the resistance being interconnected with a resonance element tuned to the intermediate frequency in such a manner that the resistance, at the intermediate frequency, has minimal attenuation for a useful signal and, at all other frequencies, terminates the output of the mixer with the system impedance.
The heterodyne stage of this invention ensures that, with minimal circuit complexity, the mixer output is loaded with the system impedance, not only in the transmission region, but also in the entire cutoff region, generally at 50 Ohms. Topographically, depending on the impedance behavior of the IF filter and a compensation element with the two circuit variantsxe2x80x94series resistance or shunt resistancexe2x80x94a total of eight circuit options results, as shown in the table of FIG. 1. Depending on whether the IF filter acts as a series circuit (low impedance in the transmission region, high impedance in the cutoff region) or as a parallel circuit (high impedence in the transmission region, low impedence in the cutoff region), and depending on whether a resonance element working in cooperation with the resistance acts as a series circuit or a parallel circuit, appropriate impedance inverters must be connected therebetween that are constructed in a known manner, for example of discrete components, and having a behavior of a xcex/4 line, that is creating a low-ohmic impedance from a high-ohmic impedance, and vice versa. The resonance element is made to be free from parasitic resonances using known measures, and its impedance level can be transformed accordingly with aid of a transformation circuit, that is, with an additional transformation circuit the impedance level of the resonance element can be multiplied or divided by a constant factor, depending on whether the ohmic resistance that terminates the separating filter output is a series resistance or a parallel resistance.